Venting arrangement for storage tanks

ABSTRACT

Petroleum storage tanks or the cargo tanks of a tank ship have an upwardly directed minimum velocity escape valve in the vent line or lines. During filling this ensures that no flammable concentration of petroleum vapors forms below the valve, e.g. at the deck level of a tank ship. A suitable valve has a tapered sleeve which cooperates with a pointed core. The sleeve lifts to provide a larger annular opening to prevent too high-pressure drop at high mass flow rates and falls to reduce the opening to keep a high velocity at lower mass flow rates.

United States Patent [72] Inventors Will Pratt Appl. No. Filed Patented Assignee Kenley;

Geoffrey Joseph Grocott, Maidstone, both of England June 25, 1970 Dec. 14, 1971 The British Petroleum Company Limited London, England Original application Oct. 8, 1968, Ser. No. 765,910, now Patent No. 3,557,740, dated Jan. 26, 197 1. Divided and this application June 25, 1970, Ser. No. 49,720

VENTING ARRANGEMENT FOR STORAGE TANKS 3 Claims, 3 Drawing Figs.

U.S. Cl 138/46, 137/508, 98/59 Int. Cl Fl5d l/00 Field of Search 138/46;

[56] References Cited UNITED STATES PATENTS 1,354,041 9/1920 Hess l37/533.17 X 1,629,961 5/1927 Nicholson l37/533.l7 X 1,977,039 10/1934 Brouse l37/533.17 X 2,295,767 9/1942 Wiggins... 137/508 X 2,911,003 11/1959 Mulick 137/508 FORElGN PATENTS 536,937 l/l932 Germany 138/46 Primary Examiner Herbert F. Ross Attorney-Morgan, Finnegan, Durham 8L Pine ABSTRACT: Petroleum storage tanks or the cargo tanks of a tank ship have an upwardly directed minimum velocity escape valve in the vent line or lines. During filling this ensures that no flammable concentration of petroleum vapors forms below the valve, e.g. at the deck level ofa tank ship.

A suitable valve has a tapered sleeve which cooperates with a pointed core. The sleeve lifts to provide a larger annular opening to prevent too high-pressure drop at high mass flow rates and falls to reduce the opening to keep a high velocity at lower mass flow rates.

VENTING ARRANGEMENT FOR STORAGE TANKS This is adivision of application Ser. No. 765,9 l0, filed Oct. 8, 1968. now U.S. Pat. No. 3,557,740, granted .Ian. 26, 1971.

This invention relates to venting arrangements for storage tanks which are used to contain'volatile liquids which produce flammable vapors. In particular it relates to venting arrangements for the cargo tanks of tank ships.

In general, the tanks of tankers which have discharged their cargos are ventilated to remove flammable vapors during an empty passage. When tankers are loading crude petroleum or products obtained therefrom there is a fire hazard in that flammable vapors arise because a volatile cargo attempts to vaporize as soon as it enters an empty tank and there is sufficient vaporization to represent a fire hazard when the air originally present in the tank and contaminated by vapor during loading its discharged from the vent pipe as loading proceeds. The discharge of this vapor constitutes a hazard in that flammable (which includes explosive) mixtures of petroleum vapors and air may form at deck level. A similar fire hazard occurs when large storage tanks of land installations are refilled with volatile, flammable materials. It is an object of the present invention to reduce this hazard.

According to the invention a storage tank is provided with an upwardly directed minimum velocity escape valve in every vent line, each minimum velocity escape valve being so constructed that dangerous quantities of flammable vapor are upwardly directed with sufficient velocity to prevent the formation of a flammable air/vapor mixtures at or below the level of the vent.

The invention is particularly suitable for application to tank ships and a tank ship according to the invention is provided with an upwardly directed minimum velocity escape valve in every vent line to its cargo tanks, each minimum velocity escape valve being so constructed that dangerous quantities of flammable vapor are upwardly directed with sufficient velocity to prevent the formation of a flammable air/vapor mixture at deck level.

Every cargo tank may be provided with its own separate vent line and in this case the minimum velocity escape valves are conveniently situated at or near deck level. Alternatively several or all the cargo tanks may share the same vent line and in this case the minimum velocity escape valves (or valve when all the cargo tanks share the same vent line) are preferably situated as high above the deck as possible, e.g., at mast-head level.

The arrangement described above ensures sufficient velocity of escape for petroleum vapors without creating undesirable pressure drops in the vent lines.

A suitable form of minimum velocity escape valve comprises a sliding member having a tapered, e.g., a frustoconical portion, which cooperates with a pointed core to define a variable aperture, the slidable member, in the use of the valve, being lifted by the flow of gas whereby sufi'icient escape velocity is maintained without creating undesirable pressure drop.

The invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. I is a diagrammatic cross section of a tank vessel,

FIG. 2 is a perspective cutaway view of a minimum velocity escape valve, and

FIG. 3 is a vertical cross section through a valve similar to the one shown in FIG. 2.

The tank ship illustrated in FIG. 1 comprises cargo tanks whose individual vents 11 are all connected to a common header [2 which connects to a common vent line in the hollow interior of the mast l3 terminating in a minimum velocity escape valve 14 situated at the mast-head. Alternatively the common header 12 may be abolished and separate minimum velocity escape valves fitted in every vent line 11.

During loading, which may take place at 5,000 tons per hour, considerable quantities of petroleum vapors are displaced-from the tanks and all of these are discharged via the minimum velocity escape valve 14 (or valves for the altemative system) which ensures a gas discharge velocity of at least 150 feet per second in an upward direction and this velocity is sufficient to ensure that dangerous quantities of vapor do not diffuse back to deck level. If the rate of loading becomes very low, and hence the rate of volume discharge of petroleum 5 vapors becomes low, the linear velocity of escape may fall well below 150 feet per second in spite of the minimum velocity escape valve. However this only happens at such low rates of loading that the total quantity of vapors discharged is not suffcient to create a hazardous concentration at deck level.

The constant velocity escape valve 14 of FIG. 1 is shown in greater detail in FIG. 2. It comprises a core member whose upper end is provided with a conical point 21. The core member 20 is surrounded by a slidable member 22 which has a frustoconical portion 23 which cooperates with the conical point 21. The whole arrangement is surrounded by a cylindrical shield 24.

During loading the slidable member 22 is lifted by the flow of gas and the greater the flow of gas the further the slidable 20 member 22 is lifted thereby providing a greater effective opening. This prevents undesirable high-pressure drops with high rates of discharge. On the other hand when the rate of discharge falls there could be a danger of petroleum vapors diffusing back to deck level but the slidable member 22 falls thereby decreasing the effective aperture. This has the effect of maintaining the pressure drop and also the velocity of discharge so that there is no danger of petroleum vapors diffusing back to deck level in spite of the decreased rate of discharge. As has been mentioned above the valve may fail to maintain high velocities for rates of discharge which are too small to constitute a hazard.

In the modification shown in FIG. 3 the slidable member 22 is attached to the shield 24 by means of extendable cylindrical bellows 25. This does not alter the basic operation of the valve but it ensures that all the gas flows through the slidable member 22. To ensure adequate centering the slidable member 22 carries tapered guide vanes 26. (Since these are thin plates parallel to the direction of flow they offer substantially no resistance.)

For protection the valve has a lid 27 (shown in the open position). When the valve is not in use, e.g., at sea, the lid is closed so that the operative parts of the valve are not open to sea or weather.

We claim:

I. A minimum velocity escape valve suitable for use in the vent lines of a tank ship; which valve comprises:

a fixed core member having a tapered end portion; and,

a hollow slidable member open at both ends surrounding said core member in spaced relation thereto and forming 5o therewith a passage for the flow of vapor therearound,

said hollow slideable member having a tapered end portion at the same end as the fixed core member, the tapered end portion of said hollow slideable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for the escape of vapor from said passage through the open tapered end portion of said hollow slideable member.

2. A minimum velocity escape valve suitable for use in the vent lines of a tank ship, which valve comprises:

a fixed core member having a tapered end portion a shield member surrounding said fixed core member in spaced relation thereto;

a hollow slideable member open at both ends disposed in the space between said fixed core member and said shield member in spaced relation to said fixed core member and fonning therewith a passage for the flow of vapor therearound, said hollow slideable member having a tapered end portion at the same end as said fixed core member, the tapered end portion of said hollow slidable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for the escape of vapor from said passage through the open tapered end portion of said hollow slideable member; and,

an extendable bellows member attached to and yieldably coupling said hollow slideable member and said shield member for ensuring that all the vapor flows through said axially, slideably movable relative to said fixed core member and having a tapered end portion at the same end as said fixed core member. the tapered end portion of said hollow slideable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for controlling the velocity of vapor escaping from said passage through the open tapered end portion of said hollow slideable member.

# i I i i 

1. A minimum velocity escape valve suitable for use in the vent lines of a tank ship; which valve comprises: a fixed core member having a tapered end portiOn; and, a hollow slidable member open at both ends surrounding said core member in spaced relation thereto and forming therewith a passage for the flow of vapor therearound, said hollow slideable member having a tapered end portion at the same end as the fixed core member, the tapered end portion of said hollow slideable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for the escape of vapor from said passage through the open tapered end portion of said hollow slideable member.
 2. A minimum velocity escape valve suitable for use in the vent lines of a tank ship, which valve comprises: a fixed core member having a tapered end portion a shield member surrounding said fixed core member in spaced relation thereto; a hollow slideable member open at both ends disposed in the space between said fixed core member and said shield member in spaced relation to said fixed core member and forming therewith a passage for the flow of vapor therearound, said hollow slideable member having a tapered end portion at the same end as said fixed core member, the tapered end portion of said hollow slidable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for the escape of vapor from said passage through the open tapered end portion of said hollow slideable member; and, an extendable bellows member attached to and yieldably coupling said hollow slideable member and said shield member for ensuring that all the vapor flows through said passage.
 3. A minimum velocity escape valve which comprises: a fixed core member having a tapered end portion; and, a hollow slideable member open at both ends surrounding said fixed core member in spaced relation thereto and forming an annular passage therewith for the flow of vapor therearound, said hollow slideable member being axially, slideably movable relative to said fixed core member and having a tapered end portion at the same end as said fixed core member, the tapered end portion of said hollow slideable member cooperating with the tapered end portion of said fixed core member to define a variable area aperture for controlling the velocity of vapor escaping from said passage through the open tapered end portion of said hollow slideable member. 